Superconducting rotating machines with stationary field coils

ABSTRACT

A machine includes a rotatable rotor assembly having a number of salient poles. The machine further includes a stationary stator assembly having concentric inner and outer stators, at least one stationary superconducting field coil and at least one stator coil. The stationary superconducting field coil is disposed between the inner and outer stators and is mounted on at least one of the inner and outer stators. The stationary superconducting field coil and the salient poles are configured relative to each other, such that when the rotor assembly is rotated relative to the stator assembly around a predetermined axis, a rotating magnetic field is produced with an airgap flux direction substantially along the predetermined axis. The interaction between the stationary superconducting field coil and the rotating poles provides the only source of a time varying magnetic flux supplied to the stator coil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent application,Ser. No. 10/792,503 filed Mar. 3, 2004 now U.S. Pat. No. 7,049,724,entitled “Superconducting Rotating Machines with Stationary Field Coilsand Axial Airgap Flux,” which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to electrical motor/generators,and more particularly to machines including superconducting windings.

At least some known superconducting electric machines include asuperconducting field coil installed on the rotor. The superconductingcoil is maintained at a temperature approaching zero degrees Kelvinusing a continuous supply of cryogenic fluid, such as, for example, butnot limited to liquid helium (He₂). If a high temperature superconductor(HTS) is used in fabricating the field coil, a cryogenic fluid such asnitrogen (N₂) may be used to achieve superconducting temperatures. Thecryogenic fluid is typically supplied to the superconducting field coilfrom a stationary cryocooler through a transfer coupling that is coupledto one end of the rotor. The transfer coupling channels the cryogenicfluid from a stationary portion to a rotating portion on the rotor. Thecryogenic fluid is then routed through a cooling loop thermally coupledto the superconducting field coil and then back to the transfer couplingfor return to the stationary cryocooler.

The superconducting field coil is subjected to thermal stresses,centrifugal stresses, and is provided with an electrical connectionthrough the rotor to power the superconducting field coil. Accordingly,designing, fabricating and operating such a rotor may be difficult. Forexample, the superconducting coils, especially HTS coils, may besensitive to mechanical strain. Specifically, because the coils arecoupled to the rotor, the coils may be subjected to centrifugal forcesthat may cause strains and degrade the performance of thesuperconductor. In addition, because the coil is maintained at acryogenic temperature, an elaborate support system may be needed tomaintain the coil in position against the centrifugal forces whilepreserving the integrity of the thermal insulation between the coil andthe parts of the rotor at ambient temperature.

It is desirable to overcome these shortcomings of the prior art.Further, it is desirable to provide a machine with improved performancecharacteristics, such as increased torque density.

BRIEF DESCRIPTION OF THE INVENTION

One aspect of the invention resides in a machine, such as a motor or agenerator, that includes a rotatable rotor assembly having a number ofsalient poles. The machine further includes a stationary stator assemblyhaving concentric inner and outer stators, at least one stationarysuperconducting field coil and at least one stator coil. The stationarysuperconducting field coil is disposed between the inner and outerstators and mounted on at least one of the inner and outer stators. Thestationary superconducting field coil and the salient poles areconfigured relative to each other, such that when the rotor assembly isrotated relative to the stator assembly around a predetermined axis, arotating magnetic field is produced with an airgap flux directionsubstantially along the predetermined axis. The interaction between thestationary superconducting field coil and the rotating poles providesthe only source of a time varying magnetic flux supplied to the statorcoil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an exemplary embodiment of asynchronous machine;

FIG. 2 is a perspective view of a machine according to an exemplaryembodiment of the invention;

FIG. 3 is a perspective view of an exemplary embodiment of a rotorassembly for the machine shown in FIG. 1;

FIG. 4 is a plan view of an exemplary embodiment of a rotor for themachine shown in FIG. 1;

FIG. 5 is a plan view of a second exemplary embodiment of a rotor forthe machine shown in FIG. 1;

FIG. 6 is a perspective view of an exemplary stator for a machineaccording to an embodiment of the invention;

FIG. 7 illustrates an exemplary configuration for an armature winding;

FIG. 8 illustrates another exemplary configuration for an armaturewinding;

FIG. 9 illustrates yet an exemplary configuration for an armaturewinding; and

FIG. 10 illustrates another exemplary configuration for armaturewindings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional view of an exemplary embodiment of anelectrical machine 100. The machine 100 may operate as a motor and/or asa generator as desired. The machine 100 includes a rotor assembly 110having a pair of rotor segments 110 a, 110 b mounted on a shaft 120. Theshaft 120 is preferably formed of a non-ferromagnetic material, such asstainless steel. The rotor segments 110 a, 110 b are adapted to rotateas the shaft rotates about its longitudinal axis 130 and arespaced-apart axially along the shaft 120. Each rotor segment 110 a, 110b includes a plurality of salient poles, such as iron poles 112 and aplurality of air poles 114. Embodiments of the rotor assembly 110 androtor segments are described in detail below with reference to FIGS. 4and 5.

The rotor assembly 110 is substantially enclosed within a stationaryhousing 140. The housing 140 rotatably supports the rotor assembly 110.The housing 140 is of a substantially cylindrical configuration. Astator assembly 150 is also supported within the housing 140 and isstationary relative to the housing 140. The stator assembly 150 includesa stator 160 having stator windings 165 (“armature windings” 165, forexample as shown in FIGS. 7-10) and a stationary field coil 170preferably made of a superconducting material. Preferably, the fieldcoil 170 is mounted on the stator 160. The stator windings 165 may beformed of conventional or superconducting materials. Embodiments ofstator assemblies are described in detail below with reference to FIG.6.

The field coil 170 is mechanically decoupled from the rotor assembly110. The field coil 170 is fabricated from a superconducting materialsuch that, when cooled to superconducting temperatures, the field coil170 exhibits substantially zero resistance to electrical current flow.The field coil 170 and the salient poles 112 are configured relative toeach other such that a rotating magnetic field with a substantiallyaxial airgap flux is produced when the rotor segments 110 a, 1110 b arerotated related relative to the stator 160 around a predetermined axis.In a preferred embodiment where the rotor segments are mounted on theshaft 120, the airgap flux direction is substantially parallel to theaxis 130 of the shaft 120.

Thus, the rotor and the stator are offset axially with the field coilbeing stationary relative to the stator. The poles of the rotor areformed on a planar surface that is substantially perpendicular to therotational axis of the rotor. Further, the superconducting field coil ispositioned in plane that is axially offset from the plane of the polesof the rotor. With the rotation of the rotor through a magnetic fieldgenerated by the field coil, a rotating magnetic field is produced. Therotating magnetic field has an airgap flux substantially in the axialdirection in the region of the stator.

In operation, the machine 100 may operate as an electrical generator oras a motor. When the machine 100 operates as a generator, the shaft 120and the rotor assembly 110 are rotated about the longitudinal axis 130of the shaft 120. The rotation of the shaft 120 and the rotor assembly110 may be performed by applying a torsional force coupled to the shaft120. The superconducting coil 170 is cooled to a temperature below itsT_(c) temperature and an electrical current is supplied to thestationary superconducting field coil 170 and the coil 170 acts as astationary magnetomotive force (MMF) source which interacts with arotating permeance wave of the rotating poles of the rotor to produce arotating AC magnetic field. The rotating magnetic field has an airgapflux directed substantially axially along the longitudinal axis 130 ofthe shaft 120 and is magnetically coupled to the stator windings 165which allows electrical power to be generated. When the machine 100operates as a motor, electrical power is provided to the machine 100 togenerate the rotating magnetic field and to cause the rotor assembly 110to rotate relative to the stator 160, which in turn rotates the shaft120.

In the exemplary embodiment, the field coil 170 is stationary relativeto the housing 140, while the rotor assembly 110 rotates relative to thehousing 140 such that a relative difference in rotational speed betweenthe rotor assembly 110 and the magnetic field generated by the fieldcoil 170 is the rotational speed of rotor 110.

FIG. 2 illustrates a three dimensional view of the machine 100 accordingto certain embodiments of the present invention where the statorassembly 150 comprises inner 162 and outer stators 164. In accordancewith one embodiment, the superconducting field coil 170 is mounted on anouter surface 163 of a ring shaped inner stator 162. A ring shaped outerstator 164 is located concentrically around the inner stator 162, suchthat the coil 170 is located between the inner 162 and outer 164stators. In accordance with another embodiment, the coil 170 is mountedon an inner surface 169 of the outer stator 164, such that the coil 170is located between the inner 162 and outer 164 stators. In accordancewith yet another embodiment, two superconducting field coils 170 areemployed, with one of the coils 170 being mounted on an outer surface163 of inner stator 162, and the other coil 170 being mounted on aninner surface 169 of the outer stator 164, such that the coils 170 arelocated between the inner 162 and outer 164 stators.

The superconducting coil 170 is maintained at a temperature below thecritical temperature of the superconducting material forming the coil170. For example, the superconducting coil 170 may be maintained at atemperature approaching zero degrees Kelvin using a continuous supply ofcryogenic fluid, such as, for example, but not limited to liquid helium(He₂). If a high temperature superconductor (HTS) is used in fabricatingthe field coil, a cryogenic fluid such as nitrogen (N₂) may be used toachieve superconducting temperatures. The cryogenic fluid is typicallysupplied to the superconducting field coil from a stationary cryocooler.Any suitable cooling fluid devices, such as cooling fluid tubes orconduits may be provided in the stator assembly 150 to cool thesuperconducting coil 170.

For embodiments employing superconducting armature windings 165, thearmature windings 165 are maintained at temperatures below the criticaltemperature of the superconducting material forming the windings 165.For example, superconducting armature windings 165 may be maintained ata temperature approaching zero degrees Kelvin using a continuous supplyof cryogenic fluid, such as, for example, but not limited to, liquidhelium (He₂). If a high temperature superconductor (HTS) is used infabricating the armature coil, a cryogenic fluid such as nitrogen (N₂)may be used to achieve superconducting temperatures. The cryogenic fluidis typically supplied to the superconducting armature coils from astationary cryocooler. Any suitable cooling fluid devices, such ascooling fluid tubes or conduits may be provided in the stator assembly150 to cool the superconducting coils 165.

The rotor segments 110 a and 110 b comprising salient poles 112 and airpoles 114 are located axially on either side of the stators 162, 164, asshown in FIG. 2. A plurality of such rotor/stator arrangements can bestacked axially along the shaft 120 for long machines. The armaturewindings are not shown in FIG. 2 for clarity.

FIG. 3 illustrates a three dimensional view of the rotor assembly 110mounted on a shaft 120. Preferably, the rotor assembly contains tworotor segments. As illustrated in FIG. 3, the rotor assembly 110 isformed by placing two similar rotor segments 110 a, 110 b in an axiallyspaced-apart configuration. The rotor segments 110 a, 110 b aresufficiently separated to allow a stator assembly to residetherebetween.

FIG. 4 is a plan view of an exemplary rotor segment 200 that may be usedwith motor 100 of FIG. 1. The rotor segment 200 includes a centralportion 210 for engaging the shaft (see FIG. 1). This central portion210 may be adapted to accommodate fixtures or adapters which secure therotor segment 200 to the shaft.

The rotor segment 200 is preferably formed of a disk-shaped base 250that is divided into one or more concentric rings. Preferably, the base250 is divided into a plurality of concentric rings, such as two rings220, 230. In a preferred embodiment, the base 250 is made of aferromagnetic material to allow flux to travel therewithin. Each ring isprovided with a plurality of spaced-apart ferromagnetic poles 240 formedon a surface of the base 250. In a preferred embodiment, the salientferromagnetic poles 240 are formed with iron plates mounted on thesurface of the base 250. In another embodiment, the salient poles 240are formed of laminated iron. In yet another embodiment, the salientpoles 240 and the base 250 are of a unitary construction.

Space between the ferromagnetic poles 240 forms air gaps, or air poles.The ferromagnetic poles 240 in adjacent rings are preferably offset byone pole pitch either within or between the concentric rings.Preferably, the salient poles 240 and the air poles therebetween areeither annular sector shaped or trapezoidal shaped. Thus, a plurality ofpoles is formed in each ring such that the poles are circumferentiallyspaced apart.

FIG. 5 illustrates an alternative embodiment of a rotor segment. Therotor segment 500 is formed of a unitary segment 520 made of aferromagnetic material. The segment 500 is shaped to form a centralportion 510 adapted to securely engage the shaft. Further, the rotorsegment 500 includes perimeter cutouts 530 and internal cutouts 540. Thecutouts 530, 540 form two rings of air poles separating ferromagneticpoles. Thus, a magnetic configuration is achieved similar to thatachieved by the embodiment illustrated in FIG. 4, where concentric ringscomprise offset salient poles separated by respective air poles 530,540.

FIG. 6 illustrates a three dimensional view of the stator assembly 150with armature slots 166 and teeth 167. The assembly 150 also includesend windings 168. The armature windings may be lap/wave windings. Thestator assembly 150 is adapted to be housed between a pair of axiallyspaced-apart rotor segments, as illustrated in FIG. 1. The stator slots166 are positioned between stator teeth along the circumferentialperimeter on each side of the inner and outer stators. Although FIG. 6shows fifty (50) slots 166 for the inner and outer stators 162, 164, inanother exemplary embodiment, each of the inner and outer stators 162,164 has forty-eight (48) slots 166.

FIGS. 7-10 depict exemplary configurations for the armature windings165. For the exemplary configuration shown in FIG. 7, stator winding 165is wound across inner and outer stators 162, 164. For the particulararrangement shown in FIG. 7, stator coils 165 are wound across four (4)stator slots 166 a-d, such that each coil 165 crosses 3 slots 166. Forthe exemplary configuration shown in FIG. 8, stator winding 165 is woundon outer stator 164. For the particular arrangement shown in FIG. 8,stator coils 165 are wound around three (3) stator teeth 167. For theexemplary configuration shown in FIG. 9, stator winding 165 is wound oninner stator 162. For the particular arrangement shown in FIG. 9, statorcoils 165 are wound around three (3) stator teeth 167. FIG. 10illustrates another stator winding configuration that includes twoseparate stator windings 165. For the arrangement of FIG. 10, one statorwinding 165 is wound on inner stator 162, and another stator winding 165is wound on outer stator 164. Beneficially, for the arrangement shown inFIG. 10, both the inner and outer stators 162, 164 are use to generatepower. The armature winding configurations shown in FIGS. 7-10 areillustrative and are not intended to limit the invention.

The above-described methods and apparatus provide a synchronous machinewith stationary superconducting field coils. This arrangement allows thecooling fluids to be more easily supplied to the superconducting fieldcoils. Transfer of cooling fluid from a stationary cooling system to arotating superconducting coil is eliminated. Further, theabove-described methods and apparatus provide a motor which generates anaxial airgap flux, resulting in improved torque density. Preferably,permanent magnets and DC coils may be omitted from the machine 100.

The present application is related to U.S. patent application Ser. No.10/444,253, filed May 21, 2003, titled “METHODS AND APPARATUS FORASSEMBLING HOMOPOLAR INDUCTOR ALTERNATORS INCLUDING SUPERCONDUCTINGWINDINGS,” which is incorporated herein by reference in its entirety.

Exemplary embodiments of electrical generating systems are describedabove in detail. The systems are not limited to the specific embodimentsdescribed herein, but rather, components of each system may be utilizedindependently and separately from other components described herein.Each system component can also be used in combination with other systemcomponents.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A machine, comprising: a rotatable rotor assembly comprising aplurality of salient poles; and a stationary stator assembly comprisingconcentric inner and outer stators, at least one stationarysuperconducting field coil and at least one stator coil, wherein thestationary superconducting field coil is disposed between the inner andouter stators and mounted on an outer surface of the inner stator;wherein the stationary superconducting field coil and the salient polesare configured relative to each other such that when the rotor assemblyis rotated relative to the stator assembly around a predetermined axis,a rotating magnetic field is produced with an airgap flux directionsubstantially along the predetermined axis, and wherein the interactionbetween the stationary superconducting field coil and the rotating polesprovides the only source of a time varying magnetic flux supplied to thestator coil.
 2. The machine of claim 1, wherein the at least one statorcoil comprises a superconducting stator coil.
 3. The machine of claim 1,wherein the at least one stator coil is wound on the inner stator. 4.The machine of claim 1, wherein the at least one stator coil is wound onthe outer stator.
 5. The machine of claim 1, wherein the stationarystator assembly comprises at least two stator coils, wherein a first ofthe stator coils is wound on the inner stator, and wherein a second ofthe stator coils is wound on the outer stator.
 6. A machine comprising:a rotatable rotor assembly comprising a plurality of salient poles; anda stationary stator assembly comprising concentric inner and outerstators, at least one stationary superconducting field coil and at leastone stator coil, wherein the stationary superconducting field coil isdisposed between the inner and outer stators and is mounted on an innersurface of the outer stator, wherein the stationary superconductingfield coil and the salient poles are configured relative to each othersuch that when the rotor assembly is rotated relative to the statorassembly around a predetermined axis, a rotating magnetic field isproduced with an airgap flux direction substantially along thepredetermined axis, and wherein the interaction between the stationarysuperconducting field coil and the rotating poles provides the onlysource of a time varying magnetic flux supplied to the stator coil. 7.The machine of claim 6, wherein the at least one stator coil comprises asuperconducting stator coil.
 8. The machine of claim 6, wherein the atleast one stator coil is wound on the inner stator.
 9. The machine ofclaim 6, wherein the at least one stator coil is wound on the outerstator.
 10. The machine of claim 6, wherein the stationary statorassembly comprises at least two stator coils, wherein a first of thestator coils is wound on the inner stator, and wherein a second of thestator coils is wound on the outer stator.
 11. A machine comprising: arotatable rotor assembly comprising a plurality of salient poles; and astationary stator assembly comprising concentric inner and outerstators, two stationary superconducting field coils and at least onestator coil, wherein one of the stationary superconducting field coilsis mounted on an outer surface of the inner stator and another of thestationary superconducting field coils is mounted on an inner surface ofthe outer stator wherein the stationary superconducting field coils andthe salient poles are configured relative to each other such that whenthe rotor assembly is rotated relative to the stator assembly around apredetermined axis, a rotating magnetic field is produced with an airgapflux direction substantially along the predetermined axis, and whereinthe interaction between the stationary superconducting field coils andthe rotating poles provides the only source of a time varying magneticflux supplied to the stator coil.
 12. The machine of claim 11, whereinthe at least one stator coil comprises a superconducting stator coil.13. The machine of claim 11, wherein the at least one stator coil iswound on the inner stator.
 14. The machine of claim 11, wherein the atleast one stator coil is wound on the outer stator.
 15. The machine ofclaim 11, wherein the stationary stator assembly comprises at least twostator coils, wherein a first of the stator coils is wound on the innerstator, and wherein a second of the stator coils is wound on the outerstator.
 16. A machine comprising: a rotatable rotor assembly comprisinga plurality of salient poles; and a stationary stator assemblycomprising concentric inner and outer stators, at least one stationarysuperconducting field coil and at least one stator coil, wherein thestationary superconducting field coil is disposed between the inner andouter stators and mounted on at least one of the inner and outerstators, wherein the at least one stator coil is wound across the innerand outer stator, wherein the stationary superconducting field coils andthe salient poles are configured relative to each other such that whenthe rotor assembly is rotated relative to the stator assembly around apredetermined axis, a rotating magnetic field is produced with an airgapflux direction substantially along the predetermined axis, and whereinthe interaction between the stationary superconducting field coils andthe rotating poles provides the only source of a time varying magneticflux supplied to the stator coil.
 17. The machine of claim 16, whereinthe at least one stator coil comprises a superconducting stator coil.18. A machine, comprising a shaft adapted to rotate about a longitudinalaxis of said shaft; a rotor assembly rotationally engaged with saidshaft, said rotor assembly comprising a plurality of salient poles, saidsalient poles being spaced apart circumferentially; and a statorassembly comprising concentric inner and outer stators, a plurality ofarmature windings and at least one stationary superconducting fieldcoil, wherein the stationary superconducting field coil is locatedbetween the inner and outer stators and mounted on an outer surface ofthe inner stator; wherein the stationary superconducting field coil andthe salient poles are configured relative to each other such when therotor assembly is rotated relative to the stator assembly around apredetermined axis, a rotating magnetic field is produced with an airgapflux direction substantially along the predetermined axis, and whereinthe interaction between the stationary superconducting field coil andthe rotating poles provides the only source of a time varying magneticflux supplied to the armature windings.
 19. The machine of claim 18,wherein the armature windings comprise superconducting armaturewindings.
 20. The machine of claim 18, wherein the armature windings arewound on the inner stator.
 21. The machine of claim 18, wherein thearmature windings are wound on the outer stator.
 22. The machine ofclaim 18, wherein at least one of the armature windings is wound on theinner stator, and wherein at least one of the armature windings is woundon the outer stator.
 23. A machine comprising a shaft adapted to rotateabout a longitudinal axis of said shaft; a rotor assembly rotationallyengaged with said shaft, said rotor assembly comprising a plurality ofsalient poles, said salient poles being spaced apart circumferentially;and a stator assembly comprising concentric inner and outer stators, aplurality of armature windings and at least one stationarysuperconducting field coil, wherein the stationary superconducting fieldcoil is located between the inner and outer stators and is mounted on aninner surface of the outer stator, wherein the stationarysuperconducting field coil and the salient poles are configured relativeto each other such when the rotor assembly is rotated relative to thestator assembly around a predetermined axis, a rotating magnetic fieldis produced with an airgap flux direction substantially along thepredetermined axis, and wherein the interaction between the stationarysuperconducting field coil and the rotating poles provides the onlysource of a time varying magnetic flux supplied to the armaturewindings.
 24. The machine of claim 23, wherein the armature windingscomprise superconducting armature windings.
 25. A machine of comprisinga shaft adapted to rotate about a longitudinal axis of said shaft; arotor assembly rotationally engaged with said shaft, said rotor assemblycomprising a plurality of salient poles, said salient poles being spacedapart circumferentially; and a stationary stator assembly comprisingconcentric inner and outer stators, a plurality of armature windings andtwo stationary superconducting field coils, wherein one of thestationary superconducting field coils is mounted on an outer surface ofthe inner stator and another of the stationary superconducting fieldcoils is mounted on an inner surface of the outer stator, wherein thestationary superconducting field coil and the salient poles areconfigured relative to each other such when the rotor assembly isrotated relative to the stator assembly around a predetermined axis, arotating magnetic field is produced with an airgap flux directionsubstantially along the predetermined axis, and wherein the interactionbetween the stationary superconducting field coil and the rotating polesprovides the only source of a time varying magnetic flux supplied to thearmature windings.
 26. The machine of claim 25, wherein the armaturewindings comprise superconducting armature windings.
 27. A machinecomprising a shaft adapted to rotate about a longitudinal axis of saidshaft; a rotor assembly rotationally engaged with said shaft, said rotorassembly comprising a plurality of salient poles, said salient polesbeing spaced apart circumferentially; and a stator assembly comprisingconcentric inner and outer stators, a plurality of armature windings andat least one stationary superconducting field coil, wherein thestationary superconducting field coil is located between the inner andouter stators and mounted on at least one of the inner and outerstators, wherein the armature windings are wound across the inner andouter stators; wherein the stationary superconducting field coil and thesalient poles are configured relative to each other such when the rotorassembly is rotated relative to the stator assembly around apredetermined axis, a rotating magnetic field is produced with an airgapflux direction substantially along the predetermined axis, and whereinthe interaction between the stationary superconducting field coil andthe rotating poles provides the only source of a time varying magneticflux supplied to the armature windings.